Attach affected nodes to lifecycle events

[Thibaut]

Problem:

• $(document).on 'page:update', ->
    $(document.body).on('click', '.selector', fn)
    $(document.body).find('.selector').click(fn2)

  … attaches duplicate event handlers on every partial replacement (since the body didn't change).

  Same with page:load.

• Turbolinks.replace() doesn't trigger the page:load event, even though it's inserting new elements into the DOM (which people may want to transform the same way they do on normal page loads).

What this PR does:

• Attach affected nodes to the page:before-unload, page:change and page:load events.

  The above snippet could be rewritten as follow:

  $(document).on 'page:update', (event) ->
    $(document.body).on('click', '.selector', fn) if event.data[0] == document.body
    $(event.data).find('.selector').click(fn2)

• Fire page:load in Turbolinks.replace() (passing the loaded nodes).

Docs soon.

cc @dhh

[Thibaut]

cc @rafaelfranca, would be great if you could take a quick look at this one as well.

[rafaelfranca]

Patch looks good. Are you going to document it on this same PR?

[Thibaut]

Thanks @rafaelfranca. I'll revamp the events docs this week-end in a separate PR.